Recording sheets

ABSTRACT

A recording sheet comprising a support and a developer layer, which develops color images on reacting with a color former, on the support containing a dimer of the compound represented by the formula: ##STR1## wherein R 1 , R 2  and R 3  each represents a hydrogen atom, an alkyl group, an aryl group, or an aralkyl group, and n is 1 to 4.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to recording sheets, more particularly, torecording sheets which develop color images on reacting with colorformers.

2. Description of the Prior Art

Recording sheets utilizing the coloring reaction between an almostcolorless basic dye precursor (hereinafter referred to as a "colorformer") and an adsorptive or acid reactive compound developing color onreacting with the color former (hereinafter referred to as a"developer"), such as clay materials, e.g., acid clay, activated clay,attapulgite, zeolite, bentonite, kaolin, etc.; organic acids, e.g.,succinic acid, tannic acid, gallic acid, alkyl-substituted phenols,bisphenol A, etc.; metal salts of aromatic carboxylic acids, e.g., zinc,aluminum or like metal salts of alkyl-substituted salicyclic acids; andacid polymers, e.g., p-phenylphenol-formaldehyde resins, etc., aregenerally well known. The term "color former" herein designates thosematerials developing color on donating an electron or receiving a protonfrom an acid, etc., and the term "developer" designates those materialswhich receive an electron or donate a proton.

Recording sheets practically utilizing the above phenomenon includepressure-sensitive copying papers as described in U.S. Pat. Nos.2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456,2,730,457, 3,418,250, etc., and heat-sensitive papers as described inJapanese Patent Publication 4160/1968, U.S. Pat. No. 2,939,009, etc. Inaddition, there is known a printing method in which a color image isobtained by feeding ink containing a color former through a medium suchas a stencil onto a sheet with a developer coated thereon, as isdescribed in German Patent Application (OLS) 1,939,624.

Recording utilizing the above phenomenon requires a physical action, forinstance, the application of pressure by means of a ballpoint pen, atypewriter, etc., or the application of heat by means of a heatingelement, electricity, etc.

A pressure-sensitive copying paper is most typical of the recordingsheets. A pressure-sensitive copying paper is obtained by dissolving acolor former in a solvent, e.g., alkylated naphthalene, alkylateddiphenyl methane, an alkylated diphenyl or, ter-phenyl, chlorinatedparaffin, etc., dispersing the resulting solution in a binder orencapsulating it, and then coating the dispersion or capsules on asupport, e.g., paper, a plastic sheet, a resin coated paper, etc. On theother hand, where the developer is dissolved in a solvent, e.g., anorganic solvent, the support is dipped in the resulting developersolution and impregnated with the same. Alternatively, the developer isdissolved or dispersed in a medium together with a binder, and coated onthe support. In general, the color former and developer are coated onthe same side or opposite sides of the support, or on the surfaces ofdifferent supports.

While these developer-coated sheets have many advantages, they stillhave several points to be improved.

For instance, where a clay material such as an activated clay or thelike is used as a developer, a color image of Crystal Violet (forexample) disappears by the action of a polar solvent, e.g., water andglycols. Where a phenol resin such as a p-phenylphenol-formaldehyderesin is used as a developer, the developer surface tends to turn yellowunder light. In this case, aldehydes such as formaldehyde and the likeare used in synthesizing the phenol resin, and the use of suchformaldehyde and the like which are toxic is undesirable. Also, where aphenol compound, such as an alkyl-substituted phenol, bisphenol A or thelike, is used as a developer, problems also occur, for example, agradual reduction in developing capability, which is considered to bedue to diffusion with time of these compounds or the diffusion thereofinto the support. In addition, in the case of these alkyl-substitutedphenols, bisphenol A, and the like, the initial developing capability isundesirably low.

These defects should be improved since they have a very bad influence onproduct value.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a color image withpolar solvent resistance.

Another object of the present invention is to reduce the yellowcoloration of a developer surface.

A further object of the present invention is to prevent a reduction indeveloping capability due to the passage of time.

Still another object of the present invention is to increase developingcapability.

A still further object of the present invention is to produce arecording sheet meeting the above requirements.

These objects are attained by using, as a developer, a dimer of thecompound represented by the formula (I) shown below: ##STR2## whereinR₁, R₂ and R₃ each represents a hydrogen atom, an alkyl group(preferably containing 1 to 30 carbon atoms, more preferably 1 to 18carbon atoms), an aryl group (e.g., phenyl, naphthyl), which may beunsubstituted or substituted with an alkyl group (preferably having 1 to30 carbon atoms, more preferably 1 to 18 carbon atoms) or a halogen atom(e.g., chlorine, bromine or iodine) or an aralkyl group (e.g., where thearyl moiety is phenyl or naphthyl and the alkyl moiety preferably has 1to 30 carbon atoms, more preferably 1 to 18 carbon atoms), n is 1 to 4,and the OH group may be present at either the meta-position or thepara-position relative to the ##STR3## moiety, the para-position beingpreferred.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a recording sheet comprising a support,and a developer layer overlaid on the support and capable of developinga color image on reacting with a color former, wherein the developerlayer contains as a developer a dimer of the compound represented by theformula (I).

The recording sheet of this invention has advantages in thatdisappearance of the colored substance due to the presence of a polarsolvent does not occur, (e.g., a polar solvent such as water), a yellowcoloration on irradiation with light, e.g., sunlight, is low, and thereduction in developing capability with the passage of time is small.

On method of synthesizing developers used in the present invention is asfollows: ##STR4## R₁, R₂, R₃ and n are as described above.

The pyrolysis of the compound represented by formula (IV) is carried outin the presence of an oxide, hydroxide, alcoholate or phenolate of analkali metal or an alkaline earth metal under anhydrous conditions.

The dimer of the compound represented by formula (I) is obtained byheating at least one compound of formula (I) at a temperature of 15° to130° C in the presence of an acid catalyst.

In this way, the dimer of the present invention can be synthesized withsafety and ease as compared to phenol resins.

The synthesis of the dimer of the present invention is described ingreater detail in Japanese Patent Application (OPI) 30852/75, BritishPat. No. 903,062 and German Patent 1,004,168.

Typical examples of the alkenyl phenols represented by formula (I) arep-isopropenyl phenol, 2-(p-hydroxyphenyl)-2-butene,2-(p-hydroxyphenyl)-2-pentene, α-(p-hydroxyphenyl)-styrene,1-(p-hydroxyphenyl)-1-isobutene, p-cyclohexenylphenol,2-(p-hydroxyphenyl)-3-methyl-2-butene, 2-(p-hydroxyphenyl)-2-hexene,2-(p-hydroxyphenyl)-2-hexadecene, m-methyl-p-isopropenylphenol,2-(o-isopropyl-p-hydroxyphenyl)-2-butene,2-(o-phenyl-p-hydroxyphenyl)-2-pentene,2-(o-tert-butyl-p-hydroxyphenyl)-2-pentene, m-isopropenylphenol,1-(p-hydroxyphenyl)-1-propylene, 2-(o-benzyl-p-hydroxyphenyl)-2-butene,2-(o-phenethyl-p-hydroxyphenyl)-2-butene, etc.

In the synthesis of the dimer of the present invention, monomer andoligomers such as trimers, etc., in addition to the dimer, may bepresent. Such a mixture can be employed as the color developer of thepresent invention with the dimer being preferably present in amount ofabout 60% by weight or higher, more preferably 80% by weight or higher,with the trimer being preferably present in an amount of 20% by weightor less and the monomer being preferably present at most in an amount ofabout 5% by weight or less.

The recording sheet of the present invention is produced by coating asolution obtained by dissolving the developer (which may containmonomers and oligomers such as trimers, etc., in addition to dimers) inan organic solvent, to which a binder and an inorganic pigment may beadded as desired or necessary, on a support or by impregnating thesupport with the above solution. In accordance with another method, thedeveloper of the present invention is dispersed in water, and theresulting dispersion, to which a binder and an inorganic pigment, etc.,may be added as desired or necessary, is coated on a support. In thiscase, the addition of silicone oil, octyl alcohol, etc., for thepurposes of defoaming or inhibiting foaming, and the addition of a smallamount of other additives as are conventionally used in the art do notinterfere with the effect of the present invention. A suitable weightratio of the binder to the solid contents (including dimer and pigment)preferably ranges from about 1:1 to 50:1 and more preferably 5:1 to 40:1and a suitable weight ratio of the pigment to the dimer is preferablyabout 0.1:1 to 100:1, most preferably 0.5:1 to 50:1, and most preferably1:1 to 10:1.

Additives which can be used in combination with the developer includeoil-adsorbing inorganic pigments (clays, e.g., kaolin, talc, bentonite,acid clay, activated clay, agalmatolite, etc.; metal oxides, e.g., zincoxide, titanium oxide, zeolite, etc.; silicic acid salts other thanclays, e.g., anhydrous silicic acid, aluminum silicate, zinc silicate,etc., and the like), metal salts of aromatic carboxylic acids (forexample, zinc 3,5-di-tert-butyl salicylate, zinc 3-cyclohexylsalicylate, and the like), phenol resins (for example, novolak typephenol-aldehyde polymers, and the like), etc.

These compounds are described in U.S. Pat. Nos. 2,972,547, 3,501,331,3,554,781, 3,625,736, 3,619,238, 3,669,711, 3,427,180, 3,455,721,3,516,845, 3,634,121, 3,672,935, 3,732,120, 3,856,553, 3,772,052,3,934,070, 3,874,895, 3,864,299, etc.

In the case that the developer of the present invention is dissolved inan organic solvent, the following organic solvents are exemplary ofthose which can be used: alcohols such as methanol, ethanol, butanol,and the like; esters such as ethyl acetate, butyl acetate, ethyllactate, n-butyl lactate, ethyl glycolate, n-butyl β-oxypropionate,isobutyl β-oxypropionate, isobutyl β-oxypropionate, n-propylβ-oxypropionate, methyl α-hydroxyisobutyrate, ethylα-hydroxy-n-butyrate, and the like; ketones such as acetone, methylethyl ketone, methyl isobutyl ketone, and the like; hydrocarbons such asbenzene, toluene, xylene, and the like; etc.

In this case, to increase the absorptivity of the developer organicsolvent, inorganic pigments such as white carbon, talc, titanium oxide,calcium carbonate, calcium hydroxide, and the like can be added.Moreover, in this case, as binders, polymethyl methacrylate,polyacrylates, polyvinyl acetate, vinyl acetate-vinyl chloridecopolymers, polyvinyl butyral, polystyrene, linear saturated polyesterresins, ethyl cellulose, cellulose acetate, nitrocellulose, and the likeare dissolved therewith, and the resulting solution is coated on asupport such as paper, a plastic sheet, a resin coated paper, and thelike. A suitable thickness of the support is preferably 10 to 500 μ andmore preferably 20 to 200 μ.

On the other hand, where the developer of the present invention is usedas a water system, it is pulverized (e.g., preferably to a size of 0.1to 100 μ and more preferably to a size of 1 to 100 μ) and dispersed inwater. Alternatively the developer is dissolved in a solvent immisciblewith water, such as ethyl acetate, benzene, toluene, and the like in aproportion of 3 to 80% of the developer, based on the solvent weight,and then the resulting solution is suspended in water and coated. Inthis case, inorganic pigments such as white carbon, talc, kaolin,agalmatolite, titanium oxide, calcium carbonate, calcium hydroxide, andthe like can be added thereto, and as binders, water-soluble naturalpolymers such as proteins, e.g., gelatin, albumin, casein, and the like,celluloses, e.g., carboxymethyl cellulose, hydroxyethyl cellulose, andthe like, saccharose, e.g., agar, sodium alginate, carboxymethyl starch,gum arabic, and the like; water-soluble synthetic polymer compounds suchas polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylic acid,polyacrylamide, and the like; latexes such as acrylate copolymerlatexes, vinyl acetate based latexes, styrene-butadiene copolymerlatexes, and the like can be added thereto as desired. The compositionso obtained is coated on a support such as paper, a plastic sheet, aresin coated sheet, etc.

In any coating composition, if used, the amount of the binder added isabout 1 to about 50 parts, preferably 5 to 40 parts, and moreparticularly 10 to 30 parts, per 100 parts by weight of the solidscontent of the developer, inorganic pigment, and the like. Where theamount of the binder used is small, insufficient film strength isobtained, whereas an excess of the binder causes a reduction indeveloping capability, which is not desired from the viewpoint of cost.

On the other hand, the inorganic pigment is typically, when used, addedin an amount of about 10 to about 10,000 parts by weight per 100 partsby weight of the developer, preferably 50 to 5,000 parts by weight, andmore preferably 100 to 1,000 parts by weight.

Moreover, if desired, the developer of the present invention can be usedin combination with conventional developers such as acid clay, activatedclay, phenol-formaldehyde resins, metal salts of aromatic carboxylicacids and the like. Where conventional developers are employed with thedeveloper of the present invention, a suitable weight ratio of theconventional developer to the developer of the present invention is 0:1to 100:1 and preferably 1:1 to 20:1.

Coating of the coating composition of the present invention wherein anorganic solvent is used as a medium is carried out by coating orprinting methods such as flexo printing method, gravure printing method,etc., using a printing machine. On the other hand, with a water systemcoating solution, wherein water is used as a medium, the coating isapplied by methods which are conventionally used in this art, such asair knife coating, roll coating, blade coating, size press coating, etc.The coating amount is as the amount of the developer, (dimer) about 0.1to about 5 g/m², preferably 0.2 to 3 g/m², and especially preferably 0.3to 2 g/m².

Below about 0.1 g/m² in coating amount, insufficient developingcapability is obtained, while the upper limit is determined mainly byproduct economics rather than performance.

Color formers which react with the developer used in the recording sheetof the present invention are not especailly limited and any conventionalcolor formers can be employed. Representative examples of these colorformers are triarylmethane compounds such as3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, i.e., CrystalViolet Lactone, 3,3-bis(p-dimethylaminophenyl)phthalide,3-(p-dimethylaminophenyl)-3-(1,2-dimethylindol-3-yl)phthalide,3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide,3-(p-dimethylaminophenyl)-3-(2-phenylindol-3-yl)phthalide,3,3-bis(1,2-dimethylindol-3-yl)-5-dimethylaminophthalide,3,3-bis(1,2-dimethylindol-3-yl)-6-dimethylaminophthalide,3,3-bis(9-ethylcarbazole-3-yl)-5-dimethylaminophthalide,3,3-bis(2-phenylindol-3-yl)-5-dimethylaminophthalide,3-p-dimethylaminophenyl-3-(1-methylpyrol-2-yl)-6-dimethyl-aminophthalide,and the like; diphenylmethane compounds such as4,4'-bis-dimethylaminobenzhydrin benzyl ether, N-halo-(e.g., iodo-,chloro-, bromo-)phenyl-leucoauramine, N-2,4,5-trichlorophenylleucoauramine, and the like; xanthene compounds such as RhodamineB-anilinolactam, Rhodamine B-p-nitroanilinolactam, RhodamineB-p-chloroanilinolactam, 7-dimethylamino-2-methoxyfluoran,7-diethylamino-2-methoxyfluoran, 7-diethylamino-3-methoxyfluoran,7-diethylamino-3-chlorofluoran, 7-diethylamino-3-chloro-2-methylfluoran,7-diethylamino-2,2-dimethylfluoran,7-diethylamino-3-acetylmethylaminofluoran,7-diethylamino-3'-methylaminofluoran, 3,7-diethylaminofluoran,7-diethylamino-3-dibenzylaminofluoran,7-diethylamino-3-methylbenzylaminofluoran,3-diethylamino-7-phenylamino-6-methylfluoran,7-diethylamino-3-chloroethylmethylaminofluoran,7-diethylamino-3-diethylaminofluoran, and the like; thiazine compoundssuch as benzoyl leuco methylene blue, p-nitrobenzoylleuco methyleneblue, and the like; spiro compounds such as3-methyl-spiro-dinaphthopyran, 3-ethyl-spiro-dinaphthopyran,3,3'-dichloro-spiro-dinaphthopyran, 3-benzylspiro-dinaphthopyran,3-methyl-naphtho-(3-methoxy-benzo)-spiro-pyran,3-propyl-spiro-dibenzopyran, and the like, and mixtures thereof. Thedeveloper of the present invention is effective particularly withtriarylmethane compounds (e.g., triphenylmethane compounds) or thiazinecompounds.

The color former is typically dissolved in a solvent and encapsulated,or dispersed in a binder solution, and then coated on a support. As thesolvent used, natural or synthetic oils can be used singly or incombination with each other. Examples of the solvents are cotton seedoil, kerosene, paraffin, naphthene oil, alkylated biphenyls, alkylatedterphenyls, chlorinated paraffins, alkylated naphthalenes, and the like.

For the encapsulation, a method utilizing the coacervation of ahydrophilic colloid sol as described in U.S. Pat. Nos. 2,800,457,2,800,458, and the interfacial polymerization method as described inBritish Pat. Nos. 867,797, 950,443, 989,264, 1,091,076, etc., can beused.

The effect of the recording sheet of the present invention was confirmedusing the following color former sheet.

Microcapsules containing the color former were produced by the methoddescribed in U.S. Pat. No. 2,800,457; the details thereof will bedescribed below. Hereinafter all parts and percentages are by weightunless otherwide indicated.

Ten parts of acid-treated pigskin gelatin and 10 parts of gum arabicwere dissolved in 400 parts of water at 40° C, and 0.2 part of Turkeyred oil was added thereto. In the solution so prepared there wassuspended and dispersed 40 parts of color former oil. This color formeroil was 2% Crystal Violet lactone dissolved in diisopropyl naphthalene.

When the size of the droplet reached an average of 5 microns, theemulsification was stopped, and after water at 40° C was added to theemulsion to make 900 parts, stirring was continued. At this time,attention had to be taken so that the liquid temperature did not dropbelow 40° C. Then, 10% acetic acid was added to adjust the pH of theliquid to 4.0 - 4.2, whereupon coacervation was caused.

Stirring was further continued at 40° C, and, after 20 minutes, theliquid was cooled with ice water to gell a coacervate film depositedaround the oil droplets.

When the liquid temperature reached 20° C, 7 parts of a 37% aqueoussolution of formaldehyde was added thereto. At 10° C, a 15% aqueoussolution of sodium hydroxide was added to adjust the pH to 9. The liquidwas then heated over a period of 20 minutes (while stirring) so as toraise the temperature to 50° C.

The microcapsule dispersion so prepared was cooled to 30° C and thencoated on paper of 40 g/m² in a coating amount (solids content) of 6g/m² and dried.

In this way, a microcapsule sheet containing Crystal Violet lactone as acolor former was obtained.

EXAMPLE 1

Ten parts of a dimer of p-isopropenylphenol synthesized in accordancewith the method described in German Patent 1,004,168, and 3 parts ofethyl cellulose were dissolved in 50 parts of ethanol, and 20 parts ofkaolin (average particle size: about 5 microns) was dispersed therein.The dispersion so prepared was coated on a base paper of 50 g/m² with acoating rod in an amount (solids content) of 5 g/m² and then dried;thus, a coated paper of the present invention was obtained.

On the thus obtained coated paper of the present invention was overlaidthe capsule sheet containing Crystal Violet lactone described above,which was then pressed at 600 Kg/cm² to develop color. The color densityafter one day was measured with a spectrophotometer, and it was 0.72 at610 mμ. On exposing the coated surface to sunlight for 10 hours, almostno yellow coloration was observed.

For comparison, the same test was conducted using2,2-bis(p-hydroxyphenyl)propane (bisphenol A) in place of the dimer ofp-isopropenylphenol, and, in this case, the color density was only 0.35.

EXAMPLE 2

Forty parts of a dimer of 2-(p-hydroxyphenyl)-2-butene produced in thesame manner as in Example 1, 6 parts of a naphthalenesulfonicacid-formaldehyde condensate, and 54 parts of water were ball milled forone day. Then, 100 parts of the dimer dispersion prepared above, 160parts of kaolin, and 80 parts of a styrene-butadiene copolymer latex(solid content: 50%; carboxy content: 0.5 wt.%; styrene content: 50wt.%; butadiene content: 49.5 wt.%) as a binder were added to 500 partsof water and mixed; thus, a coating solution of the present inventionwas obtained.

This coating solution was coated on a base paper of 50 g/m² in an amount(solids) of 6 g/m² with a coating rod, and dried.

The thus obtained coated paper of the present invention was subjected tothe same test as in Example 1, and a color density of 0.78 was obtained.Furthermore, almost no yellow coloration due to sun light was observed.

For comparison, 2,2-bis(p-hydroxyphenyl)-butane was used in place of thedimer of the present invention, and, in this case, the color density wasonly 0.33.

EXAMPLE 3

Fifty parts of a dimer of 2-(p-hydroxyphenyl)-2-pentene (obtained in thesame manner as in Example 1) and 5 parts of nitrocellulose weredissolved in 100 parts of toluene, and the resulting solution was coatedon a base paper of 50 g/m² with a coating rod so that a solids contentof 1.0 g/m² was coated, and then dried. Using the thus obtained coatedpaper of the present invention, the same test as in Example 1 wasconducted. In this case, the color density was 0.65.

Furthermore, almost no yellow coloration due to sunlight was observed.

For comparison, 2,2-bis(p-hydroxyphenyl)-pentane was used in place ofthe dimer of the present invention, and in this case, the color densitywas only 0.24.

EXAMPLE 4

Using a dimer of m-methyl-p-isopropenylphenol (obtained in the samemanner as in Example 1), a developer sheet of the present invention wasobtained in the same manner as in Example 1, and, on effecting the sametest as in Example 1, a color density of 0.78 was obtained.

Furthermore almost no yellow coloration due to sunlight was observed.

For comparison, 2,2-bis(m-methyl-p-hydroxyphenyl)-propane was used, and,in this case, the color density was only 0.34.

EXAMPLE 5

Using a dimer of 1-(p-hydroxyphenyl)-1-propylene (obtained in the samemanner as in Example 1), a developer sheet of the present invention wasobtained in the same manner as in Example 1, and, on effecting the sametest as in Example 1, a color density of 0.73 was obtained. Furthermorealmost no yellow coloration due to sunlight was observed.

For comparison, 1,1-bis(p-hydrophenyl)propane was used, and in thiscase, the color density was only 0.33.

EXAMPLE 6

Using a dimer of α-(p-hydroxyphenyl)-styrene (obtained in the samemanner as in Example 1), a developer sheet of the present invention wasobtained in the same manner as in Example 2, and, on effecting the sametest as in Example 1, a color density of 0.71 was obtained. Furthermore,almost no yellow coloration due to sunlight was observed.

For comparison, 1-phenyl-1,1-bis(p-hydroxyphenyl)-ethane was used, and,in this case, the color density was only 0.34.

EXAMPLE 7

Using a dimer of 2-(p-hydroxyphenyl)3-methyl-2-butene (obtained in thesame manner as in Example 1), a developer sheet of the present inventionwas obtained in the same manner as in Example 2, and, on effecting thesame test as in Example 1, a color density of 0.80 was obtained.Furthermore, almost no yellow coloration due to sunlight was observed.

For comparison, 2,2-(p-hydroxyphenyl)-3-methyl-butane was used, and, inthis case, the color density was only 0.38.

EXAMPLE 8

Using a dimer of 2-(p-hydroxyphenyl)-2-hexene obtained in the samemanner as in Example 1, a developer sheet of the present invention wasobtained in the same manner as in Example 2, and, on effecting the sametest as in Example 1, a color density of 0.79 was obtained. Furthermorealmost no yellow coloration due to sunlight was observed.

For comparison, 2,2-(p-hydroxyphenyl)-hexane was used, and, in thiscase, the color density was only 0.35.

From the above examples, it can be understood that the present inventionprovides excellent developer sheets.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A pressure-sensitive recording sheet comprising asupport having a developer layer on the support, said developer layerbeing capable of developing color images on reacting with a colorformer, wherein the developer layer contains as a developer a dimer ofthe compound represented by the formula: ##STR5## wherein R₁, R₂, and R₃each represents a hydrogen atom, an alkyl group, an aryl group, or anaralkyl group, and n is 1 to
 4. 2. The recording sheet according toclaim 1, wherein the alkyl group contains 1 to 30 carbon atoms.
 3. Therecording sheet according to claim 1, wherein the aralkyl group containsan alkyl moiety containing 1 to 30 carbon atoms.
 4. The recording sheetaccording to claim 1, wherein the OH group is present at the meta-orpara-position relative to the ##STR6## moiety.
 5. The recording sheetaccording to claim 1, wherein R₁ is a hydrogen atom.
 6. The recordingsheet according to claim 1, wherein R₂ is an alkyl group.
 7. Therecording sheet according to claim 1, wherein said dimer is a dimer ofthe compound having the formula ##STR7##
 8. The recording sheetaccording to claim 1, wherein R₃ is a hydrogen atom.
 9. The recordingsheet according to claim 1, wherein said dimer is a dimer of thecompound having the formula ##STR8##